A power amplifier is the component in a radio frequency transmitter that amplifies the signal to be transmitted to a power level high enough to be conducted to the output port (e.g. antenna port) of the transmitter. In systems employing digital linear modulation schemes the linearity of the power amplifier is a very important factor. Digital linear modulation means that the signal to be transmitted has a number of allowed phase- and amplitude states, whereby each allowed phase-amplitude combination represents a certain bit pattern in a bit stream used as the source of the modulation and constitutes a so-called constellation point in a phase-amplitude diagram. The power amplifier has to reproduce the phase-amplitude combinations of the signal to be transmitted so that the relative distances between the constellation points is not distorted; hence the strict requirements for linearity in the operation of the power amplifier.
A traditional linear power amplifier is a so-called Class A amplifier meaning that the amplifying component is not driven to output current cutoff at any allowed value of the driving voltage. If the driving voltage is a sinusoidal oscillation, the conduction angle of a Class A amplifier is said to be 360 degrees. The drawback of Class A amplifiers is their poor efficiency, which may theoretically be 50% but falls in most practical circuits to the order of 25 to 30%. The problem is made worse by the requirement of a large dynamic range which is typical to the mobile terminals of digital cellular radio systems. As an example we may consider a mobile telephone, where the difference between the lowest and highest values of transmitted power may be several tens of decibels. If the linear power amplifier is designed for maximal efficiency at one end (usually the high end) of the dynamic range by selecting the structural parameters of the amplifying semiconductor component in a suitable way, efficiency at the other end (the low end) of the dynamic range is bound to be poor. Correspondingly a relatively large amount of electric energy is wasted during operation in the latter end of the dynamic range. In portable radio devices like mobile telephones wasting energy means unnecessarily shortening the operating time before the next required recharging of batteries.
From the European patent application number 98660044.3, published in December 1998 and incorporated herein by reference, there is known an arrangement consisting of at least two parallel amplifier stages, of which only one is selected for use at any moment of time by setting the bias voltages of the amplifier stages to suitable values. In such a structure each of the parallel amplifier stages may have a different optimal power range which is a subset of the required dynamic range of the whole amplifier arrangement. A separate control circuit selects for use the amplifier stage whose optimal power range is closest to the momentarily required output power of the amplifier arrangement. A remarkable saving is achievable in comparison to a single linear power amplifier stage, but there remains the problem of the efficiency remaining under 50% even if continuous optimal operation is assumed. Additionally it has been shown that immediately after changing the selected amplifier branch a phase distortion occurs in the output signal, which is a detrimental effect in systems using continuous phase modulation.